1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus having the function of monitoring abnormal operation such as abnormal discharge or the like in an X-ray tube unit.
2. Description of the Related Art
As is known, a helical scan by which projection data in a wide region of a subject to be examined can be continuously and seamlessly acquired at a high speed is realized by synchronous motions, i.e., the continuous rotation of an X-ray tube, a multi-channel type X-ray detector, and the like and the movement of the table-top of a bed on which the subject is placed.
Most of the currently available X-ray computed tomography apparatuses are equipped with an interlock function of monitoring the state of an X-ray tube on the basis of a tube voltage, a tube current, a filament heating current, an intra-tube temperature, the operation state of a cool pump, and the like and stopping scan operation including X-ray irradiation as needed.
Conventionally, when this interlock function is activated to stop X-ray irradiation, the cause is sought and solved. In addition, if the operator gives no error clear instruction, the overall scan operation including X-ray irradiation is stopped.
Such an interlock function is very convenient and an indispensable function in improving safety. On the other hand, a scan must often be started all over again. In a contrast medium examination or the like, for example, a change in CT value over time is one of the most important information. If a scan is stopped halfway by the interlock function, the contrast enhancement effect is almost lost. For this reason, a contrast medium must be injected again to redo a scan from the beginning. In the above helical scan, even if the scan operation is resumed from the scan stop time, discontinuous data may be obtained due to a change in the posture of a subject to be examined or the like during the time the scan is stopped. In such a case as well, a scan must be redone from the beginning.
In general, an X-ray tube is equipped with a diaphragm device and designed to change the slice thickness by adjusting the aperture width of the diaphragm. Since the slice thickness is determined depending on a diaphragm aperture width, it is determined before data acquisition, and there is basically no degree of freedom in changing the slice thickness after data acquisition. If, therefore, tomographic images having different slice thicknesses are required, the operator must repeat a scan upon changing the diaphragm aperture width. Jpn. Pat. Appln. KOKAI Publication No. 9-215688 discloses a technique of simultaneously acquiring data having different slice thicknesses by using a dual detector. However, data are obtained at different slice positions (different slice center positions). When, therefore, tomographic images having different slice thicknesses are required at the same slice position, a scan must also be repeated.
Recently, a semiconductor detector which directly converts X-rays transmitted through a subject to be examined into electric charges has been developed, and the X-ray irradiation dose, i.e., the exposure dose, tends to decrease with an improvement in the sensitivity of the detector. However, the reduction in exposure dose has its limit. As the exposure dose becomes a predetermined amount or less, the density resolution considerably decreases, resulting in incapability of ensuring image quality suitable for diagnosis.
In contrast-medium imaging, it is important to perform a scan (a helical scan in this case; a monitor-scan is discriminated from a main-scan) at the timing a contrast medium injected into a subject to be examined flows into an imaging region. For this reason, the flow of the contrast medium must be monitored at a position upstream of the imaging region. That is, a scan is consecutively repeated at a monitoring position, and tomographic image data is reconstructed and displayed in real time from the resultant projection data.
The operator observes this tomographic image and inputs a trigger for a main-scan at the timing the degree of staining of a blood vessel of interest increases to a certain extent. Upon reception of this trigger, the apparatus stops the monitor-scan and moves the table-top to a scan start position in the imaging region. In addition, the apparatus waits for setups of scan conditions (X-ray emission conditions, helical pitch, and the like) for a main-scan from the operator, switches the output voltages of the high voltage generating unit in accordance with the setup conditions, and pre-heats the filament. After this preparation is completed, the apparatus actually starts a main-scan by giving a tube voltage and trigger. As described above, table-top movement, setups, and emission preparation are required before a main-scan is actually started.
The time required for these operations is not short. For this reason, a main-scan may start with a delay with respect to the proper timing a contrast medium begins to flow into an imaging region. In order to prevent such a situation, a trigger for a main-scan must be input at the timing when the degree of staining of a blood vessel is not very high, in consideration of the operation time. This timing determination requires expert knowledge and experience.